1. Field of the Invention
The present invention relates to a vehicular electric power generation apparatus and method, which may be used in passenger vehicles, truck vehicles and the like.
2. Description of Related Art
A recent trend involves electronically controlling an idling rotation speed of a vehicle engine. In this instance, the idling rotation speed is determined based on engine temperature the magnitude of the electric load, on/off status of an air conditioner and the like. The engine rotation is stabilized to a target idling rotation speed by feeding back the actual rotation speed of the engine during engine idling operation. The idling rotation speed of the engine is lowered to meet various social requirements, such as fuel economy, lower noise during engine idling, and environmental protection.
It became a recent problem that if the engine idling rotation speed does not stabilize, a hunting occurs, at which to higher and lower rotation speeds. The electric power output of a vehicular alternating current (a.c.) generator driven by the engine generally increases up to a certain rotation speed with an increase in its rotation speed and saturates after reaching the certain rotation speed.
Further, the driving torque of the a.c. generator may be considered as the output per rotation of the a.c. generator. Thus, the relation between the rotation speed and the driving torque is in a monotonic increase until the output saturates relative to the rotation speed, and is in a monotonic decrease after the output saturation. As a result, as shown in FIG. 3, the a.c. generator generally has a maximum driving torque Tp at a rotation speed No. The maximum driving torque changes sharply at around the rotation speed No. The increase and decrease in the engine rotation speed is controlled by an idling speed control apparatus as described above, even in the range of engine idling operation in which the output torque of the engine is not sufficient. For this reason, when the rotation speed of the a.c. generator at the time of engine idling rotation and the rotation speed No, at which the a.c. generator requires a maximum driving torque Tp, generally coincide at the rotation speed No, a response delay of control or the like causes a hunting in which the engine idling rotation speed falls far below and rises far above the rotation speed No at which the driving torque Tp of the a.c. generator reaches a maximum.
When the hunting occurs, sound of changes in the rotation speed of the engine and in rotation speeds of a number of devices driven by the engine become noisy, and changes in vibration caused by the changes in the rotation speed becomes sensible. Thus, users of vehicles are likely to feel uncomfortable. This will result in an unacceptable problem in recent passenger compartments, which have increased quietness.
It is therefore desired in view of the foregoing problems to provide a vehicular a.c. generator, which requires a lower driving toque and does not have sharp maximum driving torque characteristics.
For decreasing the driving torque, reduction in inertial moment of a rotary body by down-sizing, improvement in efficiency by improving cooling and the like have been attained. However, those are not effective to restrict hunting of rotation speed.
Further, as disclosed in JP-B2-6-38720, it is known to regulate an energization current under a cold state for restricting the cold state driving torque to the same level as the hot state driving torque, because the a.c. generator has a larger driving torque in the cold state than in the hot state. However, this method is still incapable of suppressing occurrence of hunting, because the a.c. generator requires the maximum driving torque in the hot state.
Further, as disclosed in JP-A-10-4698, although it is proposed to flatten the maximum driving torque characteristics by restricting an electric power generation output at around the maximum driving torque, the generated power output is decreased.